As is well-known, a photosensitive resin has been used for a recent flexographic plate, and this photosensitive resin is generally composed of an elastomeric binder, at least one monomer and a photopolymerization initiator. An original printing plate using this photosensitive resin is a plate provided with a layer of the photosensitive resin on a support substrate.
In production of a flexographic plate using this original printing plate, first a film (mask) having a negative pattern of an image such as a text and a picture to be printed is placed on the photosensitive resin layer of this original printing plate, and the photosensitive resin layer is then exposed to a chemical ray through this mask. A portion exposed with the chemical ray is cured as a result of a photopolymerization reaction. Thereafter, an uncured portion is washed out with a developer, and then a relief pattern in accordance with the image remains. As a result, the flexographic plate is produced. In the flexography, an ink is put on a top surface of the relief pattern, which is compressed on a printing medium such as paper to carry out printing.
A negative film used as the negative mask is produced by the use of a photographic technique commonly used, i.e., a photochemical technique using silver halide, and thus the production is expensive and steps thereof are complicated. As an alternative to the photographic film, a negative mask which may be produced more simply and inexpensively has been proposed (Patent Document 1: JP-P-2003-330158 A). This negative mask may be produced by the following process: a layer for bearing an ink which may be referred to as an ink receiving layer is provided on a surface of a transparent support substrate such as a polyester film, and printing a negative pattern on this ink bearing layer by an inkjet printer with an ultraviolet light-absorptive ink.
This negative mask may be produced simply and inexpensively, and can be sufficiently used as an alternative to the conventional negative film. However, the negative mask also has similar problems to those of the conventional negative film to be improved, such as follows:
(i) A size of the negative mask may easily alter due to changes of temperature and humidity because the negative mask is composed of a plastic film. Thus, even if the same negative mask is used, each pattern-forming procedure including the light exposure and development of the photosensitive resin layer may result in different accuracy of the printing plate products, due to difference in time and environment for performing the procedure.
(ii) In the pattern forming step, a substance such as dust which inhibits light incidence tends to come in between the negative mask and the photosensitive resin layer. If such a substance has come in, a pattern image obtained after the exposure and the development may be disturbed, which may disgrace printing quality of the printing plate.
In order to solve such problems, Patent Documents 2 and 3 (JP-P-H5-11445 A, and JP-P-H10-10709 A, respectively) disclose direct printing with a UV absorbable ink composition on the surface of the photosensitive resin layer using a printing apparatus such as an inkjet printer and a laser printer, whereby printing a negative pattern.